1. Field of the Invention
The invention relates to austenitic heat-resistant cast steels, and more particularly to austenitic heat-resistant cast steels having excellent thermal fatigue characteristics.
2. Description of the Related Art
In order for austenitic heat-resistant cast steels to have excellent thermal fatigue characteristics at 950° C. or more, for example, they must have excellent high-temperature strength properties and excellent toughness from room temperature to elevated temperatures. Temperature-resistant cast steels for resolving such a challenge are described in Japanese Patent Application Publication No. 2004-269979 (JP-A-2004-269979) and Japanese Patent Application Publication No. 2002-194511 (JP-A-2002-194511). JP-A-2004-269979 discloses temperature-resistant cast steels which, based on a total of 100 mass %, include 0.5 to 1.5% of carbon (C), 0.01 to 2% of silicon (Si), 3 to 20% of manganese (Mn), 0.03 to 0.2% of phosphorus (P), 3 to 20% of nickel (Ni), 10 to 25% of chromium (Cr), 0.5 to 4% of niobium (Nb) and 0.1% or less of aluminum (Al), and which also include a total of 1.5 to 6% of one or both of molybdenum (Mo) and tungsten (W), with the balance being primarily iron (Fe).
In iron-based austenitic heat-resistant cast steels, carbon is effective for increasing high-temperature strength and improving castability, and acts as an austenite phase-stabilizing element. Chromium is effective for improving the high-temperature strength, but lowers the toughness when added in a large amount. Moreover, the presence of nickel together with chromium helps increase the high-temperature strength, stabilizing the austenite phase. In light of the above, among iron-based austenitic heat-resistant cast steels according to the related art, use is frequently made of steels containing about 0.3 to 0.8% of carbon, about 10 to 25% of chromium and about 10 to 21% of nickel. In the Japanese industrial Standards (JIS), such steels are designated as, for example, SCH12 and SCH22.
In recent years, nickel has become an increasingly scarce element, in addition to which the cost has skyrocketed. For these reasons, even in austenitic heat-resistant cast steels, the tendency has been to seek lower nickel levels. However, at a low nickel content, the matrix structure is unable to achieve a uniform austenite phase, as a result of which the high-temperature strength decreases. Hence, it is not easy to lower the nickel level while maintaining high-temperature strength characteristics. Adding elements such as vanadium, molybdenum, tungsten and niobium is effective for enhancing the strength. However, these elements have a tendency to lower the toughness, thus making it difficult to achieve both high-temperature strength and toughness.